1. Field of the Invention
The present invention relates to a digital amplifier apparatus and a method of resetting a digital amplifier apparatus. The invention is suitable for application to digital amplifiers (D-class amplifiers) of, for example, Pulse-Width Modulation (PWM) type.
2. Description of the Related Art
Hitherto, with audio apparatuses incorporating a digital amplifier, the user may push the reset or mute button, thus performing a reset or mute operation, while the speaker is generating sound. Then, the speaker is forcedly stopped, ceasing to generate sound.
At the time when the user performs the reset operation, the application of a gate voltage to the gate of the Metal Oxide Semiconductor-Field Effect Transistor (MOS-FET), i.e., the switching element of the PWM-signal amplifying unit, is stopped, setting the MOS-FET into a high-impedance state. The speaker connected to the output of the MOS-FET is thereby made to stop generating sound.
More specifically, in the PWM-signal amplifying unit 1 shown in FIG. 2, an PWM signal P1(+) and a PWM signal P1(−), both received from a PWM-signal generating circuit (not shown) connected to the input of the unit 1, are supplied to the gates of transistors Tr1 and Tr2, MOS-FETs, respectively. Therefore, the transistors Tr1 and Tr2 are alternately turned on and off.
Similarly, in the PWM-signal amplifying unit 1, a PWM signal P2(+) and a PWM signal P2(−), both received from the PWM-signal generating circuit, are supplied to the gates of transistors Tr3 and Tr4, MOS-FETs, respectively. Therefore, the transistors Tr3 and Tr4 are alternately turned on and off.
Thus, in the PWM-signal amplifying unit 1, when the transistor Tr1 and the transistor Tr2 are turned on and off, respectively, and the transistor Tr3 and the transistor Tr4 are turned off and on, respectively, a drive current iA that corresponds to the PWM output OUT1 of the transistors Tr1 and Tr2 is supplied via a low-pass filter LPF1 to a speaker SP.
In the PWM-signal amplifying unit 1, too, when the transistor Tr3 and the transistor Tr4 are turned on and off, respectively, and the transistor Tr1 and the transistor Tr2 are turned off and on, respectively, a drive current iB that corresponds to the PWM output OUT2 of the transistors Tr3 and Tr4 is supplied via a low-pass filter LPF2 to the speaker SP.
That is, in the PWM-signal amplifying unit 1, potential differences d1 and d2 develop between the PWM output OUT1 and the PWM output OUT2 as shown in FIGS. 3A and 3B if the PWM output OUT1 and PWM output OUT2 have different duty ratios. The drive currents iA and iB, which correspond to the PWM outputs OUT1 and OUT2, respectively, therefore flow to the speaker SP. As a result, the speaker SP generates sound.
In the PWM-signal amplifying unit 1, no potential differences develop between the PWM output OUT1 and PWM output OUT2 as shown in FIG. 3C if the PWM output OUT1 and PWM output OUT2 have the same duty ratio of 50% and are completely in the same phase. If this is the case, neither drive current iA nor drive current iB flows to the speaker SP. Hence, the speaker SP stops generating sound and assumes an anacoustic state. Hereinafter, a signal or signals indicated to be at or to have an anacoustic level may be considered as a signal or signals which are utilized to enable the speaker to be in an anacoustic state. Likewise, a signal or signals indicated to not have an anacoustic level may be considered as a signal or signals which do not enable the speaker to be in an anacoustic state.
A digital amplifier based on a technical concept similar to this is known (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-214998). In this amplifier, to set the audio output of the speaker SP into an acoustic state, the input audio signal is multiplied by a coefficient “0,” converting the same to a digital audio signal at the anacoustic level. From the digital audio signal, a PWM-signal generating unit connected to the input of the speaker SP generates a PWM signal having a duty ratio of 50%.
In the digital amplifier thus configured, a digital audio signal, which does not have the anacoustic level as the signal already input to the digital filter, is input to the PWM-signal generating unit before the user performs an operation to set the sound volume to zero. Hence, the anacoustic PWM-signal generating unit connected to the output of the PWM-signal generating unit generates a PWM signal having a duty ratio of 50%, thereby setting the digital audio signal into the anacoustic state.
Therefore, the digital amplifier disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2004-214998 needs to have not only an anacoustic PWM-signal generating unit that is connected to the output of the PWM-signal generating unit, but also a coefficient-detecting unit for detecting whether an operation has been performed to set the sound volume to zero. The digital amplifier will inevitably be complex in structure and have a large size.
The present invention has been made in consideration of the foregoing. The purpose of the invention is to propose a digital amplifier apparatus and a method of resetting a digital amplifier apparatus, each capable of stopping an audio output means, without generating noise with a simple configuration when a resetting is performed.